Intense research to improve the characteristics of .gamma.-ray detectors together with major developments in electronics and computer technology has turned Positron Emission Tomography (PET) and Single Photon Emission Computer Tomography (SPECT) into powerful research and clinical tools. However, currently available calibration phantoms used for research and quality control of detector and camera performance do not have the same degree of sophistication. Typical calibration phantoms comprise pre-formed cavities for containing a liquid radioisotope. These cavities are supported by mechanical structures and placed within the PET and SPECT cameras. Based on their intrinsic geometry or through manual (or remote) motion of the phantom, different characteristics of the camera under study can be obtained from the resulting phantoms. Sample phantoms are described by Jaszczak in U.S. Pat. No. 4,499,375, by Smith et al. in U.S. Pat. No. 4,618,826, and by Nambu et al. in U.S. Pat. No. 5,071,602, and are advertised for purchase in the BIODEX Medical Catalog, among other sources. While phantoms such as so-called Hoffman phantoms are available in many shapes and sizes, such phantoms unfortunately require source collimation to reduce background effects and are restricted in location and distribution of the radioisotope. These limitations, among others, make the current techniques for generating phantoms less than ideal.
As known by those skilled in the art, the PET cameras and SPECT cameras calibrated by such phantoms are used to generate images of body tissues and organs for determining metabolic functions and the like. SPECT does not rely on positron emitters to function. Their operation is instead based on gamma emitting radioisotopes (single photon), although positron emitting radioisotopes have also been used in several applications. As described, for example, in U.S. Pat. No. 4,748,328 to Chang et al., the contents of which are hereby incorporated by reference, SPECT cameras only record one event per detected photon. As described, for example, in U.S. Pat. No. 4,980,552 to Cho et al., also hereby incorporated by reference, PET cameras instead record the simultaneous arrival of two photons (in coincidence) to define an event. In PET, by following the respective gamma rays back to their point of emission, the location within the patient's tissue or organ of the source of the detected gamma rays may be determined, thus providing a method of determining the path of the radioisotopes through the patient and hence useful diagnostic information such as metabolic functioning. The present invention is designed for use with PET and SPECT or with any other instrument capable of detecting gamma rays.
Prior to examination with PET or SPECT cameras, a quickly decaying radionuclide such as a radiopaque sugar derivative is administered to a patient. As the radionuclide is absorbed by the patient, it becomes distributed in accordance with the patient's unique physiological functions and/or anatomy. Then, as these radionuclides decay, positrons are emitted from the radionuclide which have a fairly broad energy distribution. Depending on the radionuclide, the end point energy can be more than 1 MeV. These very energetic particles travel a certain distance (typically a few millimeters in human tissue) before losing all of their energy and recombining with an electron to produce two oppositely directed gamma rays. Due to momentum conservation, the ejection angle for these gamma rays is slightly different from 180 degrees. The resulting pair of 511 keV gamma rays are measured by the PET camera. Generally, the image obtained with such a very small positron emitting source such as a point source will show a blurring effect due to the finite positron range. It is desired to develop a phantom which does not suffer from this problem by making the beam monoenergetic and the range for positrons at the proposed acceleration energy very small (tenths of a micrometer or less).
Radioisotopes are desirable since they may be safely administered to a patient yet emit electromagnetic radiation (gamma-rays) over a broad range, either as a direct result of nuclear decay (SPECT) or as a consequence of positron-electron annihilation (PET and SPECT). However, in order to detect the resulting gamma ray fluxes, the cameras must be very sensitive and must be regularly calibrated. An efficient phantom calibration technique is thus desirable.
The present invention operates based on the principle of "positron moderation" to replace the aforementioned prior art mechanical phantoms with electronically generated phantoms. Although the process of positron moderation in solids has been known for many years, moderation yields near 0.5% and brightness enhancement of slow positron beams was only recently demonstrated using highly sophisticated techniques such as those described by Mills, Jr. et al. in an article entitled "Solid Neon Moderator for Producing Slow Positrons," Appl. Phys. Lett., Vol. 49, pp. 1121-1123 (1986). In general, the term "positron moderation" refers to the physical processes where a high energy positron (&gt;10 keV) loses its energy to the interacting media and is finally ejected from the degrading material with relatively low energy (&lt;10-20 eV). Those skilled in the art will appreciate that energetic positrons implanted into a solid surface will reach thermal equilibrium with the lattice in a few picoseconds. Diffusion of the positrons then takes place in a larger time scale (.about.100 psec.), and different processes occur depending on the properties of the material. In a metal, annihilation with electrons is the predominant event, although, as noted by Charlton et al. in an article entitled "The Production of Low Energy Positrons and Positronium," Hyperfine Interactions, Vol. 76, pp. 97-113 (1993), positrons can be reemitted from the surface (as well as positronium) if their workfunction is negative. Charlton et al. noted that, in insulators, after cooling down below the band gap, positrons can only lose energy through phonon emission and diffusion takes place over large distances. Defects in the crystal lattice also act as trapping centers which diminish the reemission process. However, in spite of a positive affinity, positrons can be ejected from insulators if some of their energy still remains when they reach the surface. Chen et al. in an article entitled "Measurement of Positron Reemission From Thin Single-Crystal W(100) Films," Phys. Rev. B, Vol. 31, No. 7, pp. 4123-4130, Apr. 1, 1985, and Schultz et al. in an article entitled "Transmitted Positron Reemission From a Thin Single-Crystal Ni(100) Foil," Phys. Rev. B, Vol. 34, No. 1, pp. 442-444, Jul. 1, 1986, have investigated several metallic crystals and established efficiencies for slow positron reemission near 10.sup.-3.
First reports of positron moderation with efficiencies in the 10.sup.-6 -10.sup.-7 range appeared in the early 1970's, and it took more than a decade to bring that number into the 10.sup.-3 range. Positron moderation in solids has several advantages over the conventional energy selection technique offered by standard .beta.-ray spectrometers. For example, positrons are reemitted with a few eV energies. Also, the energy width of the moderated beam is very narrow (typically limited by the positrons thermal energy in the lattice), and emission takes place normal to the surface with a relatively small angular spread. These two features already make positron moderation techniques very attractive for experiments that require good energy resolution. Indeed, when process efficiency is compared, positron moderation is at least two orders of magnitude higher even for spectrometers with wide energy windows (&gt;50 keV).
Two different geometries are commonly used for positron moderation: back reemission and forward transmission. In the back reemission geometry, a sizable portion of the moderated positrons is absorbed by the primary source itself. Forward transmission-reemission, on the other hand, facilitates electromagnetic configuration for beam acceleration and focusing but has some practical limitations due to fixed foil thickness, complexity of crystal preparation, and removal of the high energy contamination from the moderated beam. Insulators such as MgO and solid rear gases are selected based on their long diffusion length for hot (eV energy) positrons, while single (W(100), Ni(100)) and polycrystalline metals present negative affinity for positrons.
Except for a few laboratories with accelerators capable of positron production through pair creation, most of the work with positron moderators has been carried out by using positron emitting radioactive sources. On the basis of half-life, intensity, branching ratios for positron emission and commercial availability, .sup.22 Na (2.6 year half-life) is the source of preference. Typical source intensities range from .mu.Ci to tens of mCi with a few groups capable of affording and handling several hundred mCi. However, despite being accelerator produced, those skilled in the art will appreciate that short-lived positron emitters such as .sup.18 F (109.8 minute half-life), .sup.11 C (20 minute half-life), .sup.13 N (10 minute half-life) and .sup.15 O (2 minute half-life) are also advantageous in that they have very high specific activities. For example, standard techniques deliver up to 1 Ci/ml of .sup.18 F from a proton irradiated .sup.18 O(H.sub.2 O) target without any further processing. Also, such positron emitters permit a short development time for new source configurations and provide a minimal residual contamination of components.
To date, only a few attempts have been made to increase positron moderation yields based on source geometry. For example, Gramsch et al. in an article entitled "Development of Transmission Positron Moderators," Appl. Phys. Lett., Vol. 51, No. 22, pp. 1862-1864 (Nov. 30, 1987) describe several thin film moderators, while Lynn et al. in an article entitled "Development of a Cone-Geometry Positron Moderator," Appl. Phys. Lett., Vol. 55, No. 1, pp. 87-89 (Jul. 3, 1989) and Khatri et al. in an article entitled "Improvement of Rare-Gas Solid Moderators By Using Conical Geometry," Appl. Phys. Lett., Vol. 57, No. 22, pp. 2374-2376 (Nov. 26, 1990) describe cylindrical and cone configurations for the positron moderator. It is desired to develop a new source geometry which provides increased conversion efficiency from high energy into low energy (few eV) positrons based on relatively standard materials and techniques. Such a source geometry also should facilitate use of the source as a positron phantom source for use in calibrating .gamma.-ray cameras without the inherent limitations of prior art phantoms and which takes advantage of advances in the efficiencies of positron moderation to form low energy positron electronic phantom sources from standard positron emission materials. The present invention has been designed to meet these needs.